1. Field of the Invention
The present invention relates to a toner, which is used to develop an electrostatic latent image in an electrophotographic method or electrostatic printing method, a developer including the toner, and a developing device and an image forming apparatus, which use the developer.
2. Description of the Related Art
A toner is used to visualize an electrostatic latent image in various image forming processes, and one known example thereof is an electrophotographic image forming process.
In an electrophotographic image forming apparatus, an image is formed by way of the respective steps of charging, exposure, developing, transferring, and fixing. First of all, in the charging step, a surface of a photoreceptor is uniformly charged, which photoreceptor serves as an image bearing member on which an electrostatic latent image is to be formed. In the exposure step, the charged surface of the photoreceptor is irradiated with light corresponding to image information so that the electrostatic latent image is formed on the surface of the photoreceptor. In the developing step, a black toner for example is attached optionally to the electrostatic latent image formed as above in order to form an image visualized with the toner (toner image) on the surface of the photoreceptor. Then, in the transferring step, the toner image is transferred onto a recording medium by electrical attraction. In case of an intermediate transfer process, the toner image is transferred onto a recording medium from an intermediate transfer member. In the final fixing step, the toner image transferred on the recording medium is molten by heat and pressured so as to be fixed on the recording medium.
As a color imaging technique has been rapidly advanced in such an electrophotographic process, a full-color image forming apparatus has been developed and now available in the market. The market scale of the full-color image forming apparatus expands according as the black-and-white image forming apparatus prevails. In reproducing colors. In the full-color image forming apparatus, it is common to use a toner in, for example, three subtractive primary colors; yellow (Y), magenta (M), and cyan (C), or tour colors composed of the above three colors and black (K). The colors are reproduced in the following procedures. Among the image forming steps, the steps of charging, exposure, developing, and transferring, are repeated. For the respective colors of C, N, Y, and K so that the toner images made of the toner of the plural colors overlap one another on a recording medium, thus forming a full-color image. Then, in the final fixing step, the above overlapping toner images are molten so as to be fixed on the recording medium. In the procedure just described, the overlapping toner images are molten so as to be mixed with each other, with the result that the colors are reproduced based on the subtractive principles.
In such a full-color electrophotographic method, development is carried out more than once and in the intermediate transferring step, a plurality of toner images different in color from each other are overlaid with one another on the intermediate transfer member, and accordingly expected chargeability, flowability, and fixing property of the toners in respective colors are extremely important factors.
Further, in order to maintain stable and favorable color reproducibility in the full-color images, for example, it is necessary to transfer a predetermined amount of toner need onto an intermediate transfer medium and recording medium from the developing step to the transferring step. An amount of attaching toner in the developing step and the transferring step is greatly influenced by charging properties such as charge rising property of the toner and environmental and temporal stability of a charge amount of toner, and flowability.
In order to improve and adjust chargeability, flowability, transferability and the like, for example, an external additive is added to a toner including a binder resin, a colorant, a charge control agent and the like. As to an amount of addition of the external additive to the toner, when downsizing of a particle size of a toner particle is progressed in order to obtain a high-definition image and thus surface area per unit weight of the toner is increased, the amount of addition thereof per unit weight of the toner tends to be increased. This is because when the surface area per unit weight of the toner is increased, adhesion of the toner is so enhanced that there arises a need to add a large amount of the external additive to maintain flowability. However, in spite of increase of the amount of addition of the external additive per unit weight of the toner, when an external additive is added under the same condition where the amount of addition of the external additive is not increased, a fall-off amount of the external additive which falls off downsized toner particles is more than a fall-off amount of the external additive which falls off non-downsized toner particles. The fallen-off external additive tends to attach to a carrier and a charging member so that the carrier and the charging member are soiled with the fallen-off external additive. When the carrier and the charging member are soiled, degrading of chargeability and filming to the photoreceptor occur.
A fall-off amount of the external additive is so dependent from a particle size in the external additive and an amount of addition thereof that there exists a method of controlling the fall-off amount of the external additive by defining the particle size and the amount of addition. Further, when an external additive is attached to a base particle with mechanical energy by using a mixer or the like, the external additive is restrained from falling off by making a mixing time longer and a circumferential velocity faster, but the external additive tends to migrate to a concave portion in a toner, so that an effect of adding an external additive is not sufficiently obtained.
Further, when a toner is molten on a surface of a recording medium so as to be fixed thereto, an external additive on the toner surface brings the toner so hard to melt that there arises a problem that fix level becomes low etc. Accordingly, it is preferable that the amount of addition of the external additive is small.
As the toner, a pulverized toner is used conventionally but it has an amorphous shape whose surface has a plurality of concavity and convexity. Since fracture plane after pulverization directly becomes the toner surface, surface composition of the toner easily becomes so nonuniform that it is difficult to control the surface condition to be uniform. Unless the surface condition is uniform, the external additive is so hard to uniformly attach to the surface of toner particle that chargeability degrades.
Further, in a toner, release agent having a low melting temperature which is effective to prevent high-temperature offset from being generated, and a low temperature-softening component that is effective to fix at a low temperature may be contained, so that exposure thereof to the toner surface causes problems of degradation of flowability and degradation of chargeability due to spent to a carrier.
Further, when toner have the amorphous shapes whose surfaces have a plurality of concavity and convexity, adhesion is so enhanced that flowability degrades and charge rising is late, with the result that there arises a problem that transfer efficiency lowers.
In view of such problems, Japanese Examined Patent Publication JP-B2 3372698 discloses a toner, in which a value of shape factor SF-1 is 120≦SF-1≦160, a value of shape factor SF-2 is 115≦SF-2≦140, the values being measured by an image analyzer for toner, a relationship between specific surface area Sb (m2/cm3) per unit volume of the toner measured by the BET method and specific surface area St (m2/cm3) per unit volume of the toner calculated from an weight average particle size on the assumption that toner are perfect sphere meets 3.0≦Sb/St≦7.0 and Sb≧St×1.5+1.5, and specific surface area Sb is 3.2 to 6.8 (m2/cm3 ). According to a toner disclosed in JP-B2 3372698, contact area between toner particles, and contact area between toner, and a photoreceptor and an intermediate transfer member are reduced, so that flowability is improved and transfer efficiency and cleaning property are enhanced.
However, a toner disclosed in JP-B2 3372698 includes pulverized toner particles, on which shell layers are not formed, surface composition of the toner particle is nonuniform, and there exists component whose polarity is the same as that of the external additive, so that the external additive easily falls off and thus soiling of a carrier and a charging member is occurred. Further, due to fall-off of the external additive, environmental and long-term stability of a charge amount of the toner is not sufficiently improved. Moreover, in a pulverized toner, degradation of chargeability due to exposure of low melting temperature component and charge instability due to nonuniformity of the surface composition is not improved.